Shape memory materials are defined by their capacity to recover a predetermined shape after significant mechanical deformation (K. Otsuka and C. M. Wayman, “Shape Memory Materials” New York: Cambridge University Press, 1998). The shape memory effect is typically initiated by a change in temperature and has been observed in metals, ceramics, and polymers. From a macroscopic point of view, the shape memory effect in polymers differs from ceramics and metals due to the lower stresses and larger recoverable strains achieved in polymers.
The basic thermomechanical response of shape memory polymer (SMP) materials is defined by four critical temperatures. The glass transition temperature, Tg, is typically represented by a transition in modulus-temperature space and can be used as a reference point to normalize temperature for some SMP systems. The melting temperature may also be a relevant transition temperature for some systems. SMPs offer the ability to vary Tg over a temperature range of several hundred degrees by control of chemistry or structure. The predeformation temperature, Td, is the temperature at which the polymer is deformed into its temporary shape. Depending on the required stress and strain level, the initial deformation Td can occur above or below Tg (Y. Liu, K. Gall, M. L. Dunn, and P. McCluskey, “Thermomechanical Recovery Couplings of Shape Memory Polymers in Flexure.” Smart Materials & Structures, vol. 12, pp. 947-954, 2003). The storage temperature, Ts, represents the temperature in which no shape recovery occurs and is equal to or below Td. The storage temperature Ts is typically less than the glass transition temperature Tg. At the recovery temperature, Tr, the shape memory effect is activated, which causes the material to substantially recover its original shape, and is typically in the vicinity of Tg. Recovery can be accomplished isothermally by heating to a fixed Tr and then holding, or by continued heating up to and past Tr. From a macroscopic viewpoint, a polymer will demonstrate a useful shape memory effect if it possesses a distinct and significant glass transition (B. Sillion, “Shape memory polymers,” Act. Chimique., vol. 3, pp. 182-188, 2002), a modulus-temperature plateau in the rubbery state (C. D. Liu, S. B. Chun, P. T. Mather, L. Zheng, E. H. Haley, and E. B. Coughlin, “Chemically cross-linked polycyclooctene: Synthesis, characterization, and shape memory behavior.” Macromolecules. vol. 35, no. 27, pp. 9868-9874, 2002), and a large difference between the maximum achievable strain, εmax, during deformation and permanent plastic strain after recovery, εp (F. Li, R. C. Larock, “New Soybean Oil-Styrene-Divinylbenzene Thermosetting Copolymers. V. Shape memory effect.” J. App. Pol. Sci., vol. 84, pp. 1533-1543, 2002). The difference εmax-εp is defined as the recoverable strain, εrecover, while the recovery ration is defined as εrecover/εmax.
The microscopic mechanism responsible for shape memory in polymers depends on both chemistry and structure (T. Takahashi, N. Hayashi, and S. Hayashi, “Structure and properties of shape memory polyurethane block copolymers.” J. App. Pol. Sci., vol. 60, pp. 1061-1069, 1996; J. R. Lin and L. W. Chen, “Study on Shape-Memory Behavior of Polyether-Based Polyurethanes. II. Influence of the Hard-Segment Content.” J. App. Pol. Sci., vol. 69, pp. 1563-1574, 1998; J. R. Lin and L. W. Chen, “Study on Shape-Memory Behavior of Polyether-Based Polyurethanes. I. Influence of soft-segment molecular weight.” J. App. Pol. Sci., vol 69, pp. 1575-1586, 1998; F. Li, W. Zhu, X. Zhang, C. Zhao, and M. Xu, “Shape memory effect of ethylene-vinyl acetate copolymers.” J. App. Poly. Sci., vol. 71, pp. 1063-1070, 1999; H. G. Jeon, P. T. Mather, and T. S. Haddad, “Shape memory and nanostructure in poly(norbornyl-POSS) copolymers.” Polym. Int., vol. 49, pp. 453-457, 2000; H. M. Jeong, S. Y. Lee, and B. K. Kim, “Shape memory polyurethane containing amorphous reversible phase.” J. Mat. Sci., vol. 35, pp. 1579-1583, 2000; A. Lendlein, A. M. Schmidt, and R. Langer, “AB-polymer networks based on oligo(epsilon-caprolactone) segments showing shape-memory properties.” Proc. Nat. Acad. Sci., vol. 98, no. 3, pp. 842-847, 2001; G. Zhu, G. Liang, Q. Xu, and Q. Yu, “Shape-memory effects of radiation crosslinked poly(epsilon-caprolactone).” J. App. Poly. Sci., vol. 90, pp. 1589-1595, 2003). One driving force for shape recovery in polymers is the low conformational entropy state created and subsequently frozen during the thermomechanical cycle (C. D. Liu, S. B. Chun, P. T. Mather, L. Zheng, E. H. Haley, and E. B. Coughlin, “Chemically cross-linked polycyclooctene: Synthesis, characterization, and shape memory behavior.” Macromolecules. Vol. 35, no. 27., pp. 9868-9874, 2002). If the polymer is deformed into its temporary shape at a temperature below Tg, or at a temperature where some of the hard polymer regions are below Tg, then internal energy restoring forces will also contribute to shape recovery. In either case, to achieve shape memory properties, the polymer must have some degree of chemical crosslinking to form a “memorable” network or must contain a finite fraction of hard regions serving as physical crosslinks.
SMPs are processed in a manner that is termed programming, whereby the polymer is deformed and set into a temporary shape. A. Lendlein, S. Kelch, “Shape Memory Polymer,” Advanced Chemie, International Edition, 41, pp. 1973-2208, 2002. When exposed to an appropriate stimulus, the SMP substantially reverts back to its permanent shape from the temporary shape. The stimulus may be, for example, temperature, magnetic field, water, or light, depending on the initial monomer systems.
The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded subject matter by which the scope of the invention is to be bound.